The REFERENCE Input resistance is 4K to 6K ohm, written in the datasheet.Is this a value between VrefX and AgndX?
Your assumption is correct, but I thought I'd give you a little more background on the architecture of this DAC. This DAC consists of a R-2R architecture, which is illustrated below. IoutX typically connects to a transimpedance amplifier stage, and connects through virtual ground, which equates to the voltage of AGNDX (IOUTX (V)=AGNDX (V)). The input resistance can then be seen as the equivalent resistance from VrefX and Agndx, which you may have noticed equates to R when simplifying the R-2R network. The value of this 'R' is the input resistance specified in the datasheet, which has a typical value of 5K for DAC8812.
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